Microspectrophotometric examination of fluorescence-Feulgen stained autoradiographs after 3HTdR labeling will be employed to define the cell population kinetics of the guinea pig L1 hepatoma under various conditions. These will include progressive growth in the ascites form and at intramuscular sites, and immune-mediated regression after a period of initial growth at subcutaneous sites. Subcutaneous tumors growing progressively in guinea pigs treated with antithymocyte serum will subsequently be studied to isolate the kinetic effects of thymocyte-mediated antitumor immunity.